1. Field of the Invention
The present invention relates to an alternating current (AC) driving light emitting device, and more particularly, to an AC driven light emitting device operable directly from AC power without requiring an AC-DC converter.
2. Description of the Related Art
A semiconductor light emitting diode (LED) is advantageous as a light source in terms of output, efficiency or reliability. Thus, the semiconductor LED is actively studied and developed as a high-output, high-efficiency light source to substitute a backlight of an illumination device or a display device.
In general, the LED is driven at a low direct current (DC). Therefore, to be driven at a normal voltage of, e.g., AC 220V, the LED requires an additional circuit such as an AC/DC converter for supplying a low DC output voltage. However, the additional circuit complicates the configuration of an LED module and may undermine efficiency and reliability when supply power is converted. Also, the additional part other than the light source increases costs and size of a product and degrades electromagnetic interference (EMI) characteristic because of a periodic component during an operation in a switching mode.
To overcome this limitation, various types of LED driving circuits, which can be driven directly at an AC voltage without using a converter, have been suggested. However, in a related art AC driven LED driving circuit, most LEDs are arranged to operate only in a specific half cycle of an AC voltage, thereby significantly increasing the number of LEDs needed for obtaining the desired quantity of light.
The necessary number of LEDs may be varied according to arrangement of the LEDs even though identical quality of light is provided. However, the related art arrangement of LEDs has very low efficiency. For example, in a representative example where the LEDs are in a reverse-parallel or bridge arrangement, the actual number of LEDs continuously emitting light is merely 50% to 60% of the total number of LEDs. That is, many LEDs are inefficiently used to attain a desired light-emission level.
Therefore, the LEDs need to be more efficiently arranged to provide equal quality of light through a smaller number of LEDs. The arrangement for improved efficiency is significantly important in terms of cost efficiency in manufacturing  selling the AC driven LED circuit.
In an actual AC driven light emitting device, unit LED cells may be connected with one another with great complexity. Thus, the interconnections and processes for forming them may be complicated and less productive. Because of the complicated interconnections between the plurality of LED cells, it is also important to design a light emitting device to be miniaturized with a high degree of integration.